Like arrays, Linked List is a linear data structure. Unlike arrays, linked list elements are not stored at contiguous location; the elements are linked using pointers.

Why Linked List?
Arrays can be used to store linear data of similar types, but arrays have following limitations.
1) The size of the arrays is fixed: So we must know the upper limit on the number of elements in advance. Also, generally, the allocated memory is equal to the upper limit irrespective of the usage.
2) Inserting a new element in an array of elements is expensive, because room has to be created for the new elements and to create room existing elements have to shifted.

For example, in a system if we maintain a sorted list of IDs in an array id[].

id[] = [1000, 1010, 1050, 2000, 2040].

 

And if we want to insert a new ID 1005, then to maintain the sorted order, we have to move all the elements after 1000 (excluding 1000).
Deletion is also expensive with arrays until unless some special techniques are used. For example, to delete 1010 in id[], everything after 1010 has to be moved.

Advantages over arrays
1) Dynamic size
2) Ease of insertion/deletion

Drawbacks:
1) Random access is not allowed. We have to access elements sequentially starting from the first node. So we cannot do binary search with linked lists efficiently with its default implementation. Read about it here.
2) Extra memory space for a pointer is required with each element of the list.
3) Not cache friendly. Since array elements are contiguous locations, there is locality of reference which is not there in case of linked lists.

Representation:
A linked list is represented by a pointer to the first node of the linked list. The first node is called head. If the linked list is empty, then value of head is NULL.
Each node in a list consists of at least two parts:
1) data
2) Pointer (Or Reference) to the next node
In C, we can represent a node using structures. Below is an example of a linked list node with an integer data.
In Java or C#, LinkedList can be represented as a class and a Node as a separate class. The LinkedList class contains a reference of Node class type.

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// A linked list node

struct Node

{

  int data;

  struct Node *next;

};

First Simple Linked List in C Let us create a simple linked list with 3 nodes.

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C

Java

Python

C#

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// A simple CPP program to introduce 

// a linked list 

#include <bits/stdc++.h>

using namespace std;

  

class Node 

{ 

    public:

    int data; 

    Node *next; 

};

  

// Program to create a simple linked 

// list with 3 nodes 

int main() 

{ 

    Node* head = NULL; 

    Node* second = NULL; 

    Node* third = NULL; 

      

// allocate 3 nodes in the heap 

    head = new Node();

    second = new Node();

    third = new Node();

  

/* Three blocks have been allocated dynamically. 

    We have pointers to these three blocks as first, 

    second and third     

    head         second         third 

        |             |             | 

        |             |             | 

    +---+-----+     +----+----+     +----+----+ 

    | # | # |     | # | # |     | # | # | 

    +---+-----+     +----+----+     +----+----+ 

      

# represents any random value. 

Data is random because we haven’t assigned 

anything yet */

      

head->data = 1; //assign data in first node 

head->next = second; // Link first node with 

                    // the second node 

      

/* data has been assigned to data part of first 

    block (block pointed by head). And next 

    pointer of first block points to second. 

    So they both are linked. 

  

    head         second         third 

        |             |             | 

        |             |             | 

    +---+---+     +----+----+     +-----+----+ 

    | 1 | o----->| # | # |     | # | # | 

    +---+---+     +----+----+     +-----+----+     

*/

      

// assign data to second node 

second->data = 2; 

  

// Link second node with the third node 

second->next = third; 

      

/* data has been assigned to data part of second 

    block (block pointed by second). And next 

    pointer of the second block points to third 

    block. So all three blocks are linked. 

      

    head         second         third 

        |             |             | 

        |             |             | 

    +---+---+     +---+---+     +----+----+ 

    | 1 | o----->| 2 | o-----> | # | # | 

    +---+---+     +---+---+     +----+----+     */    

      

third->data = 3; //assign data to third node 

third->next = NULL; 

      

/* data has been assigned to data part of third 

    block (block pointed by third). And next pointer 

    of the third block is made NULL to indicate 

    that the linked list is terminated here. 

  

    We have the linked list ready. 

  

        head     

            | 

            | 

        +---+---+     +---+---+     +----+------+ 

        | 1 | o----->| 2 | o-----> | 3 | NULL | 

        +---+---+     +---+---+     +----+------+     

      

      

    Note that only head is sufficient to represent 

    the whole list. We can traverse the complete 

    list by following next pointers. */    

  

return 0; 

}